An enthusiast recently gave me Thomas Knolls PhotoShop 0.68 dated Oct. 16, 1988 (pre-Adobe). "It even runs in OS X," he said. So I tried it.
The floppy holding the 376KB program could not be read by the new G4 Power Mac that Apple had loaned eWeek Labs, but once I plugged in a USB floppy drive, PhotoShop 0.68 opened cheerfully, showing 24-bit color and a familiar palette of tools. To be precise, it ran in Classic Mode under OS X. It also ran on a 1989 Macintosh SE/30, showing a black-and-white bit map.
In place of the familiar magnifying glass was an odd square icon for the zoom tool. There was an encircled cross hair that was not implemented (if you know its function, tell me at email@example.com). There were plenty of familiar tools as well, including levels, feather options and 18 filters; all but "convolve" survive today.
That early PhotoShop interface set standards for clarity and utility. Credit Knoll first for developing it and Adobe second for sticking with it. Credit Apple, too, whose operating system code reached back so gracefully to allow me a glimpse of the beginnings of a fine tool.